In recent years, counterfeiting or falsification of documents including currency, checks, pre-paid tickets or debit cards, credit cards, ID cards, passports, security passes, licenses and the like, and their theft and misuse has proliferated. Security systems for incorporation into such documents involve the incorporation of encoded data, either visibly or invisibly or both, into or on the document substrate and a system for reading out the encoded data to authorize use of the document. In documents for general circulation or dispensation, e.g. currency, pre-paid tickets and the like, the document incorporates a security pattern e.g. a bar code pattern hidden from view. Other personalized documents, intended to be used only by an authorized user, incorporate visible user identification and limits of use data and other data stored magnetically. Cash cards, for example, include card visible data as well as identifying data stored in a magnetic stripe and require entry of a user selected personal identification number (PIN) to be compared to the PIN entered by the user before the card is accepted and the transaction requested is authorized.
To increase the security of such documents, from counterfeiting or unauthorized use, it has also been proposed to incorporate a magnetic stripe bar code or other pattern visible on or hidden within the document substrate that is read out by a magnetic head. Bar codes formed of magnetic ink have advantages over optical bar codes, first because the pattern can be hidden from view and second because the magnetic pattern does not deteriorate with wear of the document, which can lead to smearing of the optical bar code or pattern. Systems for reading visible magnetic bar code or other codes used on checks, can also be reliably operated at high rates of speed, which is important in such high volume operations as clearing checks.
In this regard, the magnetic materials employed in magnetic inks, e.g. Fe.sub.3 O.sub.4, for use on banknotes, checks, and certain credit cards, typically have medium magnetic coercivity of 80-300 Oe. Magnetic materials for other credit cards, e.g. BaFe.sub.12 O.sub.19 or SrFe.sub.12 O.sub.19, have a high magnetic coercivity on the order of 3000 Oe. In both cases, the encoded magnetic pattern can be magnetized as the document is moved past a magnetic field, and then the magnetization pattern can be read out by an adjacent read head or spaced heads as shown, for example, in U.S. Pat. No. 4,863,196. The high remanence allows for the heads to be less sensitive. Conventional inductive heads can be used to both detect the magnetic pattern and read out the recorded code.
In a further U.S. Pat. No. 4,182,481, a series of spaced apart magnetoresistive (MR) sensor heads are employed in line with a permanent magnet for magnetizing the bar code bands before the magnetically polarized bar code bands are advanced past the MR sensor heads. The spacing of bands of the magnetic bar code pattern represents the code that is read out by the spaced apart MR heads.
It is recognized in the prior art that while the use of materials of high magnetic coercivity and remanence in magnetic bar code or authentication patterns hidden in the substrate is therefore advantageous, the persisting remanent magnetic field can be read out with comparable systems or the pattern duplicated by anhysterectic duplication. Considerable effort has been expended in schemes for blocking unauthorized duplication or decoding of the hidden bar code due to this possibility.
For additional security, it has been suggested to provide mixtures of high and relatively low coercivity magnetic materials in the same or in separate bar code bands of the bar codes and to provide more complex read heads. For example, the high coercivity bands or band materials may be magnetically polarized in different directions than the polarization of lower coercivity material bands. Such a security system is disclosed in the above-referenced '196 patent employing a plurality of read heads and a threshold comparison circuit for differentiating the bands.
In U.S. Pat. No. 4,982,076, it is also proposed that the bar code be formed with bar code bands of differing magnetic coercivity levels, as well as of a non-magnetic material as a "dummy" band that cannot be magnetized and read out magnetically. The medium and high coercivity materials are stated to be of the types listed above. The lowest coercivity materials include Fe powder, Sendust alloy powder, Mo-permalloy powder, Mn-Zn ferrite, Ni-Zn ferrite, Cu-Zn ferrite or the like.
The high coercivity bar code band is magnetized in a permanent magnetic state that is unaffected during read out. During magnetic reading of the bar code bands by a magnetic read head, the low, medium and high coercivity bar code bands are stated to be detectable, and as a result that the state of the high coercivity bar code band can be read out. Only the medium and high coercivity bands retain residual magnetization after read out that can be later detected by a magnetic viewer. Therefore, the actual code is at least partly undecipherable, since an attempt to magnetically view the bar code pattern will result in an inaccurate reproduction of only the high and medium bar code bands of the bar code pattern.